Determination of the Mobile-Hydrogen Charge State in Hydrogenated Amorphous Silicon

We find that hydrogen diffuses as H⁺, H⁰, or H^- in hydrogenated amorphous silicon depending on its location within the i-layer of a p-i-n device. We annealed a set of five p-i-n devices, each with a thin deuterium-doped layer at a different location in the i-layer, and observed the D-diffusion using secondary ion mass spectrometry (SIMS). When H-diffuses in a charged state, electric fields in the device strongly influence the direction and distance of diffusion. When D is incorporated into a device near the p-layer, almost all of the D-diffusion occurs as D⁺, and when the D is incorporated near the n-layer, most of the D-diffusion occurs as D^-. We correlate the preferential direction of D-motion at given depth within the i-layer, with the local Fermi level (as calculated by solar cell simulations), to empirically determine an effective correlation energy for mobile-H electronic transitions of 0.39 ± 0.1 eV. Using this procedure, the best fit to the data produces a disorder broadening of the transition levels of ∼0.25 eV. The midpoint between the H^0/+ and the H^0/- transition levels is ∼0.20 ± 0.05 eV above midgap.